Communication systems designed to incorporate the characteristic of communicating with many remote subscriber units for brief intervals on the same communication channel are termed multiple access communication systems. One type of communication system which can be a multiple access system is a spread spectrum system. In a spread spectrum system, a modulation technique is utilized in which a transmitted signal is spread over a wide frequency band within the communication channel. The frequency band is much wider than the minimum bandwidth required to transmit the information being sent. A voice signal, for example, can be sent with amplitude modulation (AM) in a bandwidth only twice that of the information itself. Other forms of modulation, such as low deviation frequency modulation (FM) or single sideband AM, also permit information to be transmitted in a bandwidth comparable to the bandwidth of the information itself. However, in a spread spectrum system, the modulation of a signal to be transmitted often includes taking a baseband signal (e.g., a voice channel) with a bandwidth of only a few kilohertz, and distributing the signal to be transmitted over a frequency band that may be many megahertz wide. This is accomplished by modulating the signal to be transmitted with the information to be sent and with a wideband encoding signal.
Generally, three types of spread spectrum communication techniques exist, including:
Direct Sequence
The modulation of a carrier by a digital code sequence whose bit rate is much higher than the information signal bandwidth. Such systems are referred to as "direct sequence" modulated systems. PA1 Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence. These systems are called "frequency hoppers." The transmitter jumps from frequency to frequency within some predetermined set; the order of frequency usage is determined by a code sequence. Similarly "time hopping" and "time-frequency hopping" have times of transmission which are regulated by a code sequence. PA1 Pulse-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
Hopping
Chirp
Information (i.e. the message signal) can be embedded in the spread spectrum signal by several methods. One method is to add the information to the spreading code before it is used for spreading modulation. This technique can be used in direct sequence and frequency hopping systems. It will be noted that the information being sent must be in a digital form prior to adding it to the spreading code, because the combination of the spreading code and the information, typically a binary code, involves module-2 addition. Alternatively, the information or message signal may be used to modulate a carrier before spreading it.
Thus, a spread spectrum system must have two properties: (1) the transmitted bandwidth should be much greater than the bandwidth or rate of the information being sent and (2) some function other than the information being sent is employed to determine the resulting modulated channel bandwidth. Spread spectrum communication systems can be implemented as multiple access systems in a number of different ways; one type of multiple access spread spectrum system being a direct sequence code division multiple access (DS-CDMA) system.
Multiple communication channels are allocated by assigning unique spreading code to each (and every) user in a shared frequency band. As a result, transmitted signals are in the same broad frequency band of the communication channel, but within unique portions of the broad frequency band assigned by the unique spreading codes. These unique spreading codes preferably are orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. Particular transmitted signals can be retrieved from the communication channel by despreading a signal representative of the sum of signals in the communication channel with a spreading code related to the particular transmitted signal which is to be retrieved from the communication channel. Further, when the spreading codes are orthogonal to one another, the received signal can be correlated with a particular spreading code such that only the desired signal related to the particular spreading code is enhanced while the other signals are not enhanced.
Spread spectrum, and other communication systems, are often subject to multipath distortion, in which several copies of a transmitted signal are received with different delays, gains and phases as a result of multiple radio path reflections. A type of receiver particularly well suited for reception of multipath spread spectrum signals is a RAKE receiver, which is well known in the art. The RAKE receiver is comprised of "fingers" which optimally combine the separate paths in the receiver. In general, the RAKE receiver can be analogized to a matched filter, where the path gains of each "finger," like the taps of a matched filter, need to be estimated to construct the RAKE receiver to accurately receive a transmitted signal. Since a transmitted signal is subject to many corruptions on its way to a receiver (multipath effects, Rayleigh fading, etc.), the receiver must estimate the path gains utilizing the corrupted transmitted signal. Clearly, the eventual received signal will only be as good as the path gain estimation per "finger" in the RAKE receiver.
Thus a need exists for a receiver, and particularly a RAKE receiver, that provides an accurate path gain estimate for each "finger" of the RAKE receiver without relying on the corrupt transmitted signal to make the estimate.